1. Field of the Invention
This invention relates to a system and method for simultaneous measurement of thermal conductivity and mass flow of a fluid using pyroelectric anemometers. The system and method of this invention are based upon the electrical response characteristics of pyroelectric anemometers to fluid flow.
2. Description of Prior Art
Thermal flow meters utilize the convective heat transfer between a moving fluid stream and a heated solid to measure flow rate. Thermal energy is transferred from the heated solid to the flowing fluid resulting in a decrease in the temperature of the heated solid, which decrease can be used as an indicator of flow rate. A thermal flow meter has two basic components: a heater and a transducer. The heater is used to elevate the solid temperature and the transducer detects the flow-induced change of the thermal process. The transducer converts the change of temperature or heat loss into an electronic signal that can be processed by electronic instruments. Thermal flow sensors are utilized in, for example, meteorology to determine wind velocity and direction, indoor climate control, biomedical measurements, such as respiration and blood flow, transport and process industries, and fluid dynamics research, such as wind-tunnel experimentation.
A pyroelectric anemometer is a device for measuring fluid flow comprising suitably oriented pyroelectric materials on which are deposited measuring electrodes symmetrically disposed about a deposited heater element. The pyroelectric materials have a high thermal sensitivity. The heater element is driven by an alternating current which generates an alternating thermal current at twice the current frequency. This alternating thermal current, in turn, generates an alternating voltage output at the thermal excitation frequency which depends on the velocity of the fluid flow. Upstream and downstream electrodes disposed on a pyroelectric material substrate measure the alternating charge redistribution of the pyroelectric substrate material due to the alternating heat flowing from the centrally located heating element on the substrate. When a fluid flow is present over the pyroelectric material substrate, the upstream electrode is cooled to a greater extent than the downstream electrode and, thus, its temperature is lower and, thus, the charge redistribution associated with the upstream electrode is less than the charge redistribution associated with the downstream electrode. The electrodes are connected to a differential amplifier whose output is connected to a further amplifier and an electronic meter. Means for heating the pyroelectric material substrate in a fluctuating manner to permit the necessary charge redistribution are also provided. As the temperature of the heater varies, the output of the two electrodes varies and, in addition, the amplitude and phase (relative to the input thermal signal) of their outputs is affected, depending upon whether they are upstream or downstream of the heating element when the fluid flows. The extent of the difference in the signal from the two electrodes is indicative of the flow velocity. A pyroelectric anemometer suitable for use in the system and method of this invention is taught by U.S. Pat. No. 4,332,157.
Pyroelectric anemometers have been shown to have extraordinarily high precision over a broad range of flows, particularly when compared to calorimetric type thermal flow meters such as capillary flow meters, and boundary-layer type thermal flow meters, such as hot wire/film anemometers and silicon flow meters.
U.S. Pat. No. 4,850,714 teaches an apparatus for measuring thermal conductivity of a gas using two temperature-dependent measuring resistors disposed along a bypass-type gas flow path. More particularly, the '714 patent teaches locating a second heatable and temperature-dependent measuring resistor in close proximity to a first measuring resistor, one behind the other along the gas flow path, and locating these two measuring resistors electrically in opposite arms of a measuring bridge.
An electrical thermal flow meter comprising a pair of thermistors in opposite arms of a Wheatstone bridge and an electric heater positioned in heat transfer relation with respect to the first thermistor and isolated from the second thermistor is taught by U.S. Pat. No. 3,425,277. Flowmeters are also taught by U.S. Pat. No. 4,449,401 which teaches a flow meter having a Venturi tube position within a passage that receives a portion of the airflow in the throat of which is disposed a constant temperature thermal anemometer which generates an output signal as a function of the total mass airflow through the flow meter; U.S. Pat. No. 4,916,948 which teaches thermal sensitive resistors positioned within a housing, externally and internally with respect to a pipe passage disposed within a central portion of the housing so that an accurate flow rate can be measured even when the fluid flow rate varies within a single plane; and U.S. Pat. No. 4,463,601 which teaches an airflow sensor for determining mass flow rate in a branch of a system having two flow branches which meet at a junction. However, none of the methods and apparatuses taught by these references is capable of simultaneously measuring thermal conductivity and mass flow rate of a gas.
U.S. Pat. No. 4,453,405 teaches a pyroelectric vorticimeter which is used to measure shear flow components in both the X and Y directions where the two shear flow components are electrically multiplied to provide an electronic indication of the vorticity of flow in a region of the pyroelectric substrate. A fluctuating heat input is applied to the pyroelectric substrate and two spaced conductor elements are used to sense a difference in surface charge fluctuations between the two conductor elements.
U.S. Pat. No. 4,608,865 teaches an integrated circuit pyroelectric sensor which uses two pyroelectric compositors for measuring a differential voltage across gates. The voltage and charge stored across each pyroelectric compositor is a function of temperature.
Finally, U.S. Pat. No. 3,519,924 teaches a heat-sensitive frequency-selective apparatus for measuring variable conditions of flowing fluid, such as temperature characteristics. The crystal unit is used to sense fluid velocity as a function of temperature difference sensed by the crystal unit, given a constant direction of flow and a constant density of the fluid.
None of the prior art of which we are aware teaches the use of a thermal mass flow meter for simultaneous measurement of thermal conductivity and mass flow.